Theory of Yielding, Strain Softening, and Steady Plastic Flow in Polymer Glasses under Constant Strain Rate Deformation

Chen, Kang; Schweizer, Kenneth S.

doi:10.1021/ma200436w

Cited by 112 publications

(163 citation statements)

References 66 publications

(177 reference statements)

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“…Mobility in the post-yield regime is found to be independent of previous aging time in the range of 0.5 to 1.5 hours. The postyield mobility is consistent with a linear dependence upon strain rate, although it is also consistent with the slightly sublinear dependence predicted by the theory of Chen and Schweizer 39 and observed in simulations by Riggleman et al 49 . These simulations 49 , the theory 39 , and our experiments agree that mobility is constant throughout the strain softening regime.…”

Section: Discussionsupporting

confidence: 89%

Measurement of Segmental Mobility during Constant Strain Rate Deformation of a Poly(methyl methacrylate) Glass

et al. 2014

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:We describe an apparatus for performing constant strain rate deformations of polymer glasses while simultaneously measuring the segmental mobility with an optical probe reorientation method. Poly(methyl methacrylate) glasses were deformed at T g -19 K, for local strain rates between 3.7x10 -5 and 1.2x10 -4 s -1 . In these experiments, the mobility initially increases in the pre-yield regime, by a factor of 40 to 160, as compared to the undeformed PMMA glass. The mobility then remains constant after yield, even as the stress is decreasing due to strain softening. This is consistent with the view that the sample is being pulled higher on the potential energy landscape in this regime. Higher strain rates lead to higher mobility in the post-yield regime and, for the range of strain rates investigated, mobility and strain rate are linearly correlated. We observe that thermal history has no influence on mobility after yield and that deformation leads to a narrowing of the distribution of segmental relaxation times. These last three observations are consistent with previously reported constant stress experiments on PMMA glasses. The experimental features reported here are compared to computer simulations and theoretical models.

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Section: Discussionsupporting

confidence: 89%

Measurement of Segmental Mobility during Constant Strain Rate Deformation of a Poly(methyl methacrylate) Glass

et al. 2014

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“…This previous work showed that small changes of the probe relaxation time in the post‐yield regime were due to small variations in the local strain rate and that correction for this effect yielded probe relaxation times that were constant in the post‐yield regime. The major features of the results are consistent with theory and simulations . At the lowest strain rate, the evolution of τ probe is not monotonic with strain; this is a reproducible feature of the data [see lowest strain rate data in Fig.…”

Section: Resultssupporting

confidence: 82%

“…Viewed from this perspective, the mechanical relaxation times must also show a large change in the pre‐yield regime that unfortunately was not observed in our experiments. The gradual shortening of the probe relaxation time observed here as yield is approached is qualitatively consistent with the behavior of the segmental relaxation time in computer simulations of polymer glasses and with the molecular theory of Chen and Schweizer . In contrast, the mechanical relaxation time approximately reaches its post‐yield value considerably before yield.…”

Section: Discussionsupporting

confidence: 87%

Comparison of mechanical and molecular measures of mobility during constant strain rate deformation of a PMMA glass

Bending

Ediger

2016

J Polym Sci B Polym Phys

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We performed constant strain rate deformation and stress relaxation on a poly(methyl methacrylate) glass at Tg – 19 K, utilizing three strain rates and initiating the stress relaxation over a large range of strain values. Following previous workers, we interpret the initial rate of decay of the stress during the relaxation experiment as a purely mechanical measure of mobility for the system. In our experiments, the mechanical mobility obtained in this manner changes by less than a factor of 3 prior to yield. During these mechanical experiments, we also performed an optical measurement of segmental mobility based on the reorientation of a molecular probe; we observe that the probe mobility increases up to a factor of 100 prior to yield. In the post‐yield regime, in contrast, the mobilities determined mechanically and by probe reorientation are quite similar and show a similar dependence on the strain rate. Dynamic heterogeneity is found to initially decrease during constant strain rate deformation and then remain constant in the post‐yield regime. These combined observations of mechanical mobility, probe mobility, and dynamic heterogeneity present a challenge for theoretical modeling of polymer glass deformation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1957–1967

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“…There seems to be general agreement between experiments, 21 theory, 9 and MD modeling 22−30 on the pivotal role of the amplitude of local density fluctuation, S 0 , as the primary structural variable in determining the deformation behavior of polymer glasses. The S 0 is related to the preparation protocol, thermodynamic, and loading conditions.…”

Section: ■ Introductionmentioning

confidence: 82%

Temperature, Frequency, and Small Static Stress Dependence of the Molecular Mobility in Deformed Amorphous Polymers near Their Glass Transition

Ondreáš

Jančář

2015

Macromolecules

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For the first time, we report on using dynamic mechanical analysis (DMA) to directly follow molecular mobility in PMMA and PC deformed by small constant tensile stress (σ) over the temperature interval from T g − 60 K to T g + 60 K. At T α′ > T g , a pronounced secondary α′-relaxation was observed exhibiting larger frequency shift and stronger σ-dependence compared to the main α-relaxation. After reaching its maximum at approximately the T g , the strain rate was progressively reduced at the onset of the α′-relaxation, attaining its minimum value at approximately the T α′ . Above the T α′ , the strain rate increased gradually again, suggesting that mobility has again been enhanced. We ascribe the observed variation in mobility to the strain-induced transition of segments from less mobile into more mobile domains, resulting in the alteration of conformational and orientational distribution. The macroscopic deformation (ε) dependence of the transient modulus, defined to characterize the system stiffness between the α and α′ relaxation (Γ*(ε)) and above the α′-relaxation (Ω*(ε)), collapsed to a single linear master curve for Γ*(ε) and the nonlinear Ω*(ε) master curve. Our results seem to indicate that the observed α′-relaxation may be caused by the process of intrabasin jump diffusion of small portions of the backbone chain over the characteristic length scale smaller than or equal to the dynamic Kuhn segment length (l K ). The existence of the α′-relaxation may be a manifestation of the large strain-altered chain packing associated with the polyamorphic phase transition as the process accompanying yielding in polymer glasses. ■ INTRODUCTIONPolymer glasses are generally considered random packing of low and high density nanometer scale domains bound together by intra-and intermolecular forces. 1 Because of chain connectivity, local strain affects the degree of freedom of structural units, an effect not observed in molecular glasses. 1 Macroscopic deformation and temperature both alter the potential energy landscape and modify structural disorder, segmental mobility, and dynamic heterogeneity in polymer glasses. In a single step isothermal creep photobleaching experiment and MD simulation, it was shown 2−4 that deformation-induced segmental mobility correlates with strain rate, increasing at the onset of yielding and decreasing during postyield strain hardening. The reduced mobility during strain hardening was related to the segment scale thermally activated yielding processes, 5−9 supposedly coupled with the increase in rate of nonaffine segmental displacements represented by the increase of the effective stiffness length (Kuhn length, l K ) with deformation. 10 However, using a multistep creep photobleaching experiments, Lee et al. 11 found no correlation between strain rate and mobility, suggesting that most likely the connection between any simple mechanical variable and segmental mobility is much more complex than anticipated originally, 12,13 and despite significant progress, the understanding of this phenomeno...

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Theory of Yielding, Strain Softening, and Steady Plastic Flow in Polymer Glasses under Constant Strain Rate Deformation

Cited by 112 publications

References 66 publications

Measurement of Segmental Mobility during Constant Strain Rate Deformation of a Poly(methyl methacrylate) Glass

Measurement of Segmental Mobility during Constant Strain Rate Deformation of a Poly(methyl methacrylate) Glass

Comparison of mechanical and molecular measures of mobility during constant strain rate deformation of a PMMA glass

Temperature, Frequency, and Small Static Stress Dependence of the Molecular Mobility in Deformed Amorphous Polymers near Their Glass Transition

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